1. Field of the Invention
The present invention relates to a cooling device equipped with a variable displacement type compressor.
2. Description of the Related Art
In a cooling device comprising a compressor, a condenser and an evaporator, if the cooling load becomes high, the temperature of the evaporator is increased. If the temperature of the evaporator is increased, the cooling ability is decreased, and thus it becomes impossible to sufficiently cool an object or space to be cooled.
To eliminate this problem, an air conditioning system of a vehicle has been equipped with a variable displacement type compressor. In this air conditioning system, the compressor displacement is controlled so that the temperature of the evaporator is maintined at a predetermined fixed desired temperature. That is, when the cooling load becomes high, and accordingly the temperature of the evaporator is increased, the compressor displacement is increased. As a result, since the amount of refrigerant fed into the evaporator is increased, the temperature of the evaporator is decreased. Conversely, when the cooling load becomes low, and accordingly the temperature of the evaporator is decreased, the compression displacement is decreased. As a result, since the amount of refrigerant fed into the evaporator is decreased, the temperature of the evaporator is increased.
However, when the above-mentioned fixed desired temperature of the evaporator is low, and the temperature of the outside air is low, frost adheres to the evaporator and to the conduit between the evaporator and the expansion valve. Accordingly, to prevent frost from adhering to the evaporator and the conduit, it is necessary to set the fixed desired temperature of the evaporator as high as possible. Nevertheless, when the fixed desired temperature of the evaporator is set high, when the cooling load becomes high, the capability of abruptly reducing the temperature of, for example, the driver's compartment, is decreased. That is, the so called temperature reduction characteristic is weakened. Consequently, when the fixed desired temperature of the evaporator is fixed to a certain temperature as in the above-mentioned air conditioning system, a problem occurs in that it is impossible to prevent frost from adhering to the evaporator and the conduit if a good temperature reduction characteristic is to be obtained.